Acoustic surface wave resonator

ABSTRACT

High Q performance is achieved in an acoustic surface wave resonator by the use of acoustic surface wave reflectors. The resonator input transducer and the reflectors are affixed to the propagation surface of a piezoelectric substrate member by photolithographic process. The input transducer is an interdigital structure having many long interleaving finger members. The reflectors also have long interleaving finger members and are positioned in close parallel relationship to the transducer finger members. The reflectors are electrically terminated by inductances.

Reeder eta]. I

v 22' Filed: 1 21 Appl.Nq.: 170,798

United States Patent 1191 I 1541. ACOUSTIC- SURFACE/WAVE RESONATOR [75] inventors: Thomas M. Reeder, Palo Alto;

Richard Smith, Balboa lsland, both of Calif.

[73] Assig nee: The'United States America as represented by-the Secretary of the 7 Air Force All n/1971 I v 310/9.8 1 1n1. c|.,... n 3119/04,110311 9/20, 110311 13/00 5 Field-jot Search ,.....333/82, '30, .7 2; 3 10/82, 9.8, 3l0/9.7,8.0 56 v I. J References Cited I T UNITED STATESI'PATEN'TS 3,596,211 4/1971 Dias et 51 ..3 33/72 1451' Feb; 13, 1973- 3,568,l02 3/l97 l Tscng ..333/30 Primary Examiner -Herm an Karl Saalbach Assistant Examine' rMarvinNussbaum Attorney l larry A. Herbert, Jr. et al.

' s71 ABSTRACT High Q performance is achieved in an acoustic/surface wave resonator by the use of acoustic surface wave reflectors. The resonator input transducer and the reflectors are affixed to the propagation surface of a piezoelectric substrate member by photolitho'graphic process. The input transducer is an interdigital structure having many long interleaving finger members. The reflectors also have long interleaving finger members and are positioned in close parallel relationship to thetransducer finger members. The reflectors are electrically terminated by inductances.

1 Claim, 2 Drawing Figures This invention relates to acoustic surface wave and "microelectronic devicesand inparticular to acoustic surface wave resonators and means for achieving high operation thereof.

Acoustic volume wave resonators have been in use for some .timeQThese resonators however have an upper" frequency 'limitsincelthe volume excitation device becomesimpractically thin for operation above 100 MHz. The-size, weight,-and power requirement of .acoustic volume ,wave deviceshhave prompted the recent development of acoustic surfacewave devices. In addition to overcoming the size, w.eight and power requirement limitations of acoustic-volume wave components, the acoustic surface wave devices utilize microelectronic techniques and can be incorporated into integrated circuits. High Q inductance components have not .beenachieved by integrated circuit techniques and consequently;.aneffective micro-electronic, resonatoryhas not, yetbeen developed. The presentinventionis directed-toward providing both an effective acoustic surface wave resonator andhigh Q inductancein integrated circuits.

, SUMMARY or THE INVENTION The present invention comprehendsan acoustic surface Iwave device for use in electronic resonator and I filter applications at VHF and UHF frequencies. This device, which falls into the microelectronic class, is

fabricated by photolitho'graphy on the polished surface of a piezoelectric crystal...Physically, it consists of a number of parallel metalelectrodes or comb structures which are interleavedso that voltages of opposite polarity can be appliedftoadjacent electrodes. With electric excitation acoustic stress w'avesare generated at the piezoelectric surface which travel symmetrically I away from the electrode structure. Similar electrode acoustic surface-wave resonator suitable for operation at VHF'and UHFfrequenciesr It is another object of the invention to provide an acoustic surface wave resonator having a-higher 0 than currently available:microelectronic devices of similar type.

These together with ot'her objects, features and advantagesof the invention will become more readily apparent from the following detailed description when taken in conjunction with the; illustrative embodiment inthe accompanying drawings.

DESCRIPTION or THE DRAWINGS FIG, 1 is a plan view of one presently preferred embodiment'of, the "inventionrand FIG. 2- is ansenlarged detail. of a. portion of the embodiment of FIG. 1 illustratingthe interdigital transducer and reflectorstructures.

2 DETAILED DESCRIPTION or THE PREFERRED EMBODIMENT I The resonator comprehended by the invention consists of a number of interleaved'metal electrodes which are deposited by photolithography on the polished surface of a piezoelectric crystal. Asshown in FIG. 1,

these electrodes are arranged into three parallel sections. When an RF potential from AC source 18 is applied to'the center section via terminal pads 5 and 8, acoustic stress waves are symmetrically radiated toward the other two sections. Electric inductors l6 and 17 connected between terminal pads 5-11 and 5-14 respectively provide the proper phase shift so that the stress waves are reflected by the outer sections back toward the center section, thus creating a resonant surface wave pattern. An important feature of the theory for this device is that the electric Q seen at the transducer terminals may be much higher than the 0,, of the inductors. Thus; a Q much higher than presently available with ordinary microcircuit components may be created.

FIGS. 1 and-2 illustrate in detail the structure of such -.a resonator. Substrate member 3 is a piezoelectric material such as lithium niobate which has its top surface polished to establish an acoustic-wave propagation surface. The electromagnetic wave to acoustic surface wave transducer comprises electrodes 7 and 4. These electrodesare of conductive materialand are affixed to the propagation surface by photolithographic process. Electrode 7 consists of terminal pad 8 and interdigital fingers 9. Electrode 4 consists of terminal pad 5 and interdigital fingers 6. The interdigital-fingers 9 of electrode 7 are interleaved with the central portion of interdigital finger 6 as illustrated in detail by FIG. 2-. Reflectors l0 and 13 are also of conductive material and are affixedto the propagation surface by photolithographic process. Reflectors l0 and 13 consist of terminal pads 11 and 14' and interdigital fingers 12 and 15. They also are interleaved with interdigital fingers 6' of electrode 4 as shown. Reflectors l0 and l3ar'e also electrically'inductively terminated by means of inductors l6 and 17.

The physical dimensions of the electrodes, their spacing and the electrical values of the inductors are design matters to be determined by the desired operating frequency of the resonators and other pertinent parameters.

Because of its micro circuit construction, the acoustic surface wave device herein described may be fabricated for operation from 10 to 2,000 MHzand beyond. Theoretical calculations predict a resonator Q 7 of 3,000 at 200 MHz using lithium niobate crystals; Its

novel configuration, small size, and compatibility with other microelectroniccomponents indicate that this device will find application infrequency control, wave a substrate member of piezoelectric material having a propagation surface adapted to permit the propagation of acoustic surfacewaves therealong,

a first electrode disposedon said' propagatio'n surface bers, 1

a first acoustic surface wave reflector consisting of a terminal pad having a multiplicity'of elongated strip members extending therefrom disposed on members extending said propagation surface and having its strip members in interleaved relationship within one outwardly disposed portion of the first electrode strip members,

a second acoustic surface wave reflector consisting of a terminal pad having a multiplicity of elongated strip members extending therefrom disposed on said propagation surface'and having its strip members in interleaved relationships with the other outwardly disposed portion of the first electrode strip members, first inductance means connected between the first reflector terminal pads; and I second inductance means connected between th second reflector terminal pads.

t 1 I v 

1. An acoustic surface wave resonator comprising: a substrate member of piezoelectric material having a propagation surface adapted to permit the propagation of acoustic surface waves therealong, a first electrode disposed on said propagation surface consisting of a terminal pad having a multiplicity of parallel elongated strip members extending therefrom, a second electrode consisting of a terminal pad having a multiplicity of parallel elongated strip members extending therefrom disposed on said propagation surface and having its strip members in interleaves relationship with the centrally disposed portion oF the first electrode strip members, a first acoustic surface wave reflector consisting of a terminal pad having a multiplicity of elongated strip members extending therefrom disposed on said propagation surface and having its strip members in interleaved relationship within one outwardly disposed portion of the first electrode strip members, a second acoustic surface wave reflector consisting of a terminal pad having a multiplicity of elongated strip members extending therefrom disposed on said propagation surface and having its strip members in interleaved relationships with the other outwardly disposed portion of the first electrode strip members, first inductance means connected between the first reflector terminal pads; and second inductance means connected between the second reflector terminal pads. 